This invention relates to a method and instrument design for detecting small inclusions in a transparent sheet. This invention uses a light trap in a unique way.
Detecting small (micron and submicron) inclusions in glass always has been a challenge. The difficulties associated with various practices are sensitivity, resolution, depth of focus, to name a few. Microscopy has the capability to detect inclusions down to the submicron range, yet it has an extremely narrow depth of focus and a small sampling area at high magnification. These are necessary for detecting small inclusions. If used alone, these restrictions make it next to impossible to analyze bulk glass. Diffused reflection/scattering has been used to identify inclusions. After mapping their location, the inclusion can be further determined by microscopy. Nevertheless, the detection limit for the diffused reflection/scattering approach is about 5 microns. In addition, the thickness of the glass is again somewhat restricted by the narrow depth of focus of the microscopy technique.
Small particles suspended in a fluid media, such as a liquid or gas, on the other hand, can be measured routinely by light scattering techniques. The differences between inclusions in a solid glass and particles suspended in a fluid are critical. One difference is that an inclusion in a glass is stationary. Its concentration level is normally very low, thus the signal intensity is so weak that it can hardly be distinguished from noise. Noise is the cross talk between surface detection (surface signals) and in depth detection (internal signals). In addition, the location of inclusions in glass would be valuable information. Due to the dynamic nature of the suspended particles in a fluid media, their location cannot be mapped. As a result, current existing instruments are not designed with particle location mapping capability. Nevertheless, we have found that the principle behind the measurement of particles suspended in fluid media is applicable for measurement of inclusions in solid glass.
My apparatus for detecting inclusions in a transparent sheet comprises in sequence a light source having a primary incident beam of light, at least one light trap and a transparent sheet. The transparent sheet has at least one exterior surface parallel to a horizontal axis and an interior depth with at least one inclusion therein. The inclusion intercepts the primary incident beam of light and creates a secondary radiation source forward scattered light. The light trap blocks the primary beam of light and prevents it from illuminating the exterior surface of the transparent sheet. The primary beam of light continues through the transparent sheet on a straight line path parallel to the horizontal axis and a detector position at an angle to the horizontal axis detects the secondary scattered light. Preferably, the angle is a 90xc2x0 angle perpendicular to the horizontal axis. Typically, the transparent sheet has two exterior surfaces and the apparatus has two light traps that prevent the primary beam from illuminating the exterior surfaces of the transparent sheet.
We have found that the unique arrangement of the light traps prevents cross talk between surface signals and internal signals. This is especially true in the thinner sheets of transparent media. This arrangement allows for measuring the sheet in one pass, rather than the multiple passes required for bulk glass where noise is not as much of a problem.
Our light scattering technique for size measurement is based on the fact that an illuminated particle (or inclusion) serves as a secondary radiation source in a manner which is related to its size. When illuminated with a beam of monochromatic light using a laser beam as the primary light source the angular distribution of the scattered intensity originated from the inclusion in the micron to submicron range, is a function of the following. The angular distribution of the scattered intensity is a function of scattered light and the incident beam, the wavelength of the incident light, and the index of refraction of the particle relative to that of the surrounding media.